Method for efficiently encoding and compressing ECG data optimized for use in an ambulatory ECG monitor

ABSTRACT

A method for efficiently encoding and compressing ECG data optimized for use in an ambulatory electrocardiography monitor is provided. ECG data is first encoded and compressed in a lossy process and further encoded and compressed in a lossless process. A compression ratio significantly higher than other Holter-type monitors is achieved. Requirements for storage space and power cell consumption are reduced, contributing to the long-term availability of the monitor.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional patent application is a continuation-in-part ofU.S. patent application Ser. No. 14/488,230, filed Sep. 16, 2014,pending; which is a continuation-in-part of U.S. patent application Ser.No. 14/080,725, filed Nov. 14, 2013, pending, and further claimspriority under 35 U.S.C. §119(e) to U.S. Provisional Patent applicationSer. No. 61/882,403, filed Sep. 25, 2013, the disclosures of which areincorporated by reference.

FIELD

This application relates in general to electrocardiographic monitoringand, in particular, to a method for efficiently encoding and compressingECG data optimized for use in an ambulatory electrocardiography monitor.

BACKGROUND

The first electrocardiogram (ECG) was invented by a Dutch physiologist,Willem Einthoven, in 1903, who used a string galvanometer to measure theelectrical activity of the heart. Generations of physicians around theworld have since used ECGs, in various forms, to diagnose heart problemsand other potential medical concerns. Although the basic principlesunderlying Dr. Einthoven's original work, including his naming ofvarious waveform deflections (Einthoven's triangle), are stillapplicable today, ECG machines have evolved from his original three-leadECG, to ECGs with unipolar leads connected to a central referenceterminal starting in 1934, to augmented unipolar leads beginning in1942, and finally to the 12-lead ECG standardized by the American HeartAssociation in 1954 and still in use today. Further advances inportability and computerized interpretation have been made, yet theelectronic design of the ECG recording apparatuses has remainedfundamentally the same for much of the past 40 years.

Essentially, an ECG measures the electrical signals emitted by the heartas generated by the propagation of the action potentials that triggerdepolarization of heart fibers. Physiologically, transmembrane ioniccurrents are generated within the heart during cardiac activation andrecovery sequences. Cardiac depolarization originates high in the rightatrium in the sinoatrial (SA) node before spreading leftward towards theleft atrium and inferiorly towards the atrioventricular (AV) node. Aftera delay occasioned by the AV node, the depolarization impulse transitsthe Bundle of His and moves into the right and left bundle branches andPurkinje fibers to activate the right and left ventricles.

During each cardiac cycle, the ionic currents create an electrical fieldin and around the heart that can be detected by ECG electrodes placed onthe skin. Cardiac electrical activity is then visually represented in anECG trace by PQRSTU-waveforms. The P-wave represents atrial electricalactivity, and the QRSTU components represent ventricular electricalactivity. Specifically, a P-wave represents atrial depolarization, whichcauses atrial contraction.

P-wave analysis based on ECG monitoring is critical to accurate cardiacrhythm diagnosis and focuses on localizing the sites of origin andpathways of arrhythmic conditions. P-wave analysis is also used in thediagnosis of other medical disorders, including imbalance of bloodchemistry. Cardiac arrhythmias are defined by the morphology of P-wavesand their relationship to QRS intervals. For instance, atrialfibrillation (AF), an abnormally rapid heart rhythm, can be confirmed byan absence of P-waves and an irregular ventricular rate. Similarly,sinoatrial block is characterized by a delay in the onset of P-waves,while junctional rhythm, an abnormal heart rhythm resulting fromimpulses coming from a locus of tissue in the area of the AV node,usually presents without P-waves or with inverted P-waves. Also, theamplitudes of P-waves are valuable for diagnosis. The presence of broad,notched P-waves can indicate left atrial enlargement. Conversely, thepresence of tall, peaked P-waves can indicate right atrial enlargement.Finally, P-waves with increased amplitude can indicate hypokalemia,caused by low blood potassium, whereas P-waves with decreased amplitudecan indicate hyperkalemia, caused by elevated blood potassium.

Cardiac rhythm disorders may present with lightheadedness, fainting,chest pain, hypoxia, syncope, palpitations, and congestive heart failure(CHF), yet rhythm disorders are often sporadic in occurrence and may notshow up in-clinic during a conventional 12-second ECG. Continuous ECGmonitoring with P-wave-centric action potential acquisition over anextended period is more apt to capture sporadic cardiac events. However,recording sufficient ECG and related physiological data over an extendedperiod remains a significant challenge, despite an over 40-year historyof ambulatory ECG monitoring efforts combined with no appreciableimprovement in P-wave acquisition techniques since Dr. Einthoven'soriginal pioneering work over a 110 years ago.

Electrocardiographic monitoring over an extended period provides aphysician with the kinds of data essential to identifying the underlyingcause of sporadic cardiac conditions, especially rhythm disorders, andother physiological events of potential concern. A 30-day observationperiod is considered the “gold standard” of monitoring, yet a 14-dayobservation period is currently pitched as being achievable byconventional ECG monitoring approaches. Realizing a 30-day observationperiod has proven unworkable with existing ECG monitoring systems, whichare arduous to employ; cumbersome, uncomfortable and not user-friendlyto the patient; and costly to manufacture and deploy. Still, if apatient's ECG could be recorded in an ambulatory setting over aprolonged time periods, particularly for more than 14 days, therebyallowing the patient to engage in activities of daily living, thechances of acquiring meaningful medical information and capturing anabnormal event while the patient is engaged in normal activities aregreatly improved.

The location of the atria and their low amplitude, low frequency contentelectrical signals make P-waves difficult to sense, particularly throughambulatory ECG monitoring. The atria are located posteriorly within thechest, and their physical distance from the skin surface adverselyaffects current strength and signal fidelity. Cardiac electricalpotentials measured dermally have an amplitude of only one-percent ofthe amplitude of transmembrane electrical potentials. The distancebetween the heart and ECG electrodes reduces the magnitude of electricalpotentials in proportion to the square of change in distance, whichcompounds the problem of sensing low amplitude P-waves. Moreover, thetissues and structures that lie between the activation regions withinthe heart and the body's surface alter the cardiac electrical field dueto changes in the electrical resistivity of adjacent tissues. Thus,surface electrical potentials, when even capable of being accuratelydetected, are smoothed over in aspect and bear only a general spatialrelationship to actual underlying cardiac events, thereby complicatingdiagnosis. Conventional 12-lead ECGs attempt to compensate for weakP-wave signals by monitoring the heart from multiple perspectives andangles, while conventional ambulatory ECGs primarily focus on monitoringhigher amplitude ventricular activity that can be readily sensed. Bothapproaches are unsatisfactory with respect to the P-wave and theaccurate, medically actionable diagnosis of the myriad cardiac rhythmdisorders that exist.

Additionally, maintaining continual contact between ECG electrodes andthe skin after a day or two of ambulatory ECG monitoring has been aproblem. Time, dirt, moisture, and other environmental contaminants, aswell as perspiration, skin oil, and dead skin cells from the patient'sbody, can get between an ECG electrode's non-conductive adhesive and theskin's surface. These factors adversely affect electrode adhesion andthe quality of cardiac signal recordings. Furthermore, the physicalmovements of the patient and their clothing impart variouscompressional, tensile, bending, and torsional forces on the contactpoint of an ECG electrode, especially over long recording times, and aninflexibly fastened ECG electrode will be prone to becoming dislodged.Moreover, dislodgment may occur unbeknownst to the patient, making theECG recordings worthless. Further, some patients may have skin that issusceptible to itching or irritation, and the wearing of ECG electrodescan aggravate such skin conditions. Thus, a patient may want or need toperiodically remove or replace ECG electrodes during a long-term ECGmonitoring period, whether to replace a dislodged electrode, reestablishbetter adhesion, alleviate itching or irritation, allow for cleansing ofthe skin, allow for showering and exercise, or for other purpose. Suchreplacement or slight alteration in electrode location actuallyfacilitates the goal of recording the ECG signal for long periods oftime.

Conventionally, multi-week or multi-month monitoring can be performed byimplantable ECG monitors, such as the Reveal LINQ insertable cardiacmonitor, manufactured by Medtronic, Inc., Minneapolis, Minn. Thismonitor can detect and record paroxysmal or asymptomatic arrhythmias forup to three years. However, like all forms of implantable medical device(IMD), use of this monitor requires invasive surgical implantation,which significantly increases costs; requires ongoing follow up by aphysician throughout the period of implantation; requires specializedequipment to retrieve monitoring data; and carries complicationsattendant to all surgery, including risks of infection, injury or death.

Holter monitors are widely used for extended ECG monitoring. Typically,they are often used for only 24-48 hours. A typical Holter monitor is awearable and portable version of an ECG that include cables for eachelectrode placed on the skin and a separate battery-powered ECGrecorder. The leads are placed in the anterior thoracic region in amanner similar to what is done with an in-clinic standard ECG machineusing electrode locations that are not specifically intended for optimalP-wave capture. The duration of monitoring depends on the sensing andstorage capabilities of the monitor. A “looping” Holter (or event)monitor can operate for a longer period of time by overwriting older ECGtracings, thence “recycling” storage in favor of extended operation, yetat the risk of losing event data. Although capable of extended ECGmonitoring, Holter monitors are cumbersome, expensive and typically onlyavailable by medical prescription, which limits their usability.Further, the skill required to properly place the electrodes on thepatient's chest precludes a patient from replacing or removing thesensing leads and usually involves moving the patient from the physicianoffice to a specialized center within the hospital or clinic.

U.S. Pat. No. 8,460,189, to Libbus et al. (“Libbus”) discloses anadherent wearable cardiac monitor that includes at least two measurementelectrodes and an accelerometer. The device includes a reusableelectronics module and a disposable adherent patch that includes theelectrodes. ECG monitoring can be conducted using multiple disposablepatches adhered to different locations on the patient's body. The deviceincludes a processor configured to control collection and transmissionof data from ECG circuitry, including generating and processing of ECGsignals and data acquired from two or more electrodes. The ECG circuitrycan be coupled to the electrodes in many ways to define an ECG vector,and the orientation of the ECG vector can be determined in response tothe polarity of the measurement electrodes and orientation of theelectrode measurement axis. The accelerometer can be used to determinethe orientation of the measurement electrodes in each of the locations.The ECG signals measured at different locations can be rotated based onthe accelerometer data to modify amplitude and direction of the ECGfeatures to approximate a standard ECG vector. The signals recorded atdifferent locations can be combined by summing a scaled version of eachsignal. Libbus further discloses that inner ECG electrodes may bepositioned near outer electrodes to increase the voltage of measured ECGsignals. However, Libbus treats ECG signal acquisition as themeasurement of a simple aggregate directional data signal withoutdifferentiating between the distinct kinds of cardiac electricalactivities presented with an ECG waveform, particularly atrial (P-wave)activity.

The ZIO XT Patch and ZIO Event Card devices, manufactured by iRhythmTech., Inc., San Francisco, Calif., are wearable monitoring devices thatare typically worn on the upper left pectoral region to respectivelyprovide continuous and looping ECG recording. The location is used tosimulate surgically implanted monitors, but without specificallyenhancing P-wave capture. Both of these devices are prescription-onlyand for single patient use. The ZIO XT Patch device is limited to a14-day period, while the electrodes only of the ZIO Event Card devicecan be worn for up to 30 days. The ZIO XT Patch device combines bothelectronic recordation components and physical electrodes into a unitaryassembly that adheres to the patient's skin. The ZIO XT Patch deviceuses adhesive sufficiently strong to support the weight of both themonitor and the electrodes over an extended period and to resistdisadherence from the patient's body, albeit at the cost of disallowingremoval or relocation during the monitoring period. The ZIO Event Carddevice is a form of downsized Holter monitor with a recorder componentthat must be removed temporarily during baths or other activities thatcould damage the non-waterproof electronics. Both devices representcompromises between length of wear and quality of ECG monitoring,especially with respect to ease of long term use, female-friendly fit,and quality of cardiac electrical potential signals, especially atrial(P-wave) signals.

ECG signals contain a large amount of information that requires largestorage space, large transmission bandwidth, and long transmission time.Long-term ECG monitoring further increases the amount of information tobe stored and processed. Data compression is useful in ECG applications,especially long-term monitoring. Data compression can reduce therequirement for data storage space, reduce power consumption, andextends monitoring time. ECG compression can be evaluated based oncompression ratio, signal error loss, and time of execution. A good ECGdata compression preferably should preserve the useful diagnosticinformation while compressing a signal to a smaller acceptable size.Currently, many Holter monitors use some types of compression algorithm;however, compression ratios are not satisfactory.

Therefore, a need remains for a low cost extended wear continuouslyrecording ECG monitor that is low power and storage space and datatransmission efficient, therefore contributing to long-term use.

SUMMARY

Physiological monitoring can be provided through a lightweight wearablemonitor that includes two components, a flexible extended wear electrodepatch and a reusable monitor recorder that removably snaps into areceptacle on the electrode patch. The wearable monitor sits centrally(in the midline) on the patient's chest along the sternum orientedtop-to-bottom. The ECG electrodes on the electrode patch are tailored tobe positioned axially along the midline of the sternum for capturingaction potential propagation in an orientation that corresponds to theaVF lead used in a conventional 12-lead ECG that is used to sensepositive or upright P-waves. The placement of the wearable monitor in alocation at the sternal midline (or immediately to either side of thesternum), with its unique narrow “hourglass”-like shape, significantlyimproves the ability of the wearable monitor to cutaneously sensecardiac electrical potential signals, particularly the P-wave (or atrialactivity) and, to a lesser extent, the QRS interval signals indicatingventricular activity in the ECG waveforms.

Moreover, the electrocardiography monitor offers superior patientcomfort, convenience and user-friendliness. The electrode patch isspecifically designed for ease of use by a patient (or caregiver);assistance by professional medical personnel is not required. Thepatient is free to replace the electrode patch at any time and need notwait for a doctor's appointment to have a new electrode patch placed.Patients can easily be taught to find the familiar physical landmarks onthe body necessary for proper placement of the electrode patch.Empowering patients with the knowledge to place the electrode patch inthe right place ensures that the ECG electrodes will be correctlypositioned on the skin, no matter the number of times that the electrodepatch is replaced. In addition, the monitor recorder operatesautomatically and the patient only need snap the monitor recorder intoplace on the electrode patch to initiate ECG monitoring. Thus, thesynergistic combination of the electrode patch and monitor recordermakes the use of the electrocardiography monitor a reliable andvirtually foolproof way to monitor a patient's ECG and physiology for anextended, or even open-ended, period of time.

Furthermore, the ECG data collected during the long-term monitoring arecompressed through a two-step compression algorithm executed by theelectrocardiography monitor. The algorithm generates a compression ratiosignificantly higher than other Holter-type monitors. Requirement forstorage space and power cell consumption are reduced, contributing tothe long-term availability of the monitor and efficient transmission ofrecorded data post-processing.

One embodiment provides a computer-implemented method for encoding andcompressing electrocardiography values. A series of electrocardiographyvalues is obtained. A plurality of bins are defined, each bin comprisinga lower threshold ECG value, an upper threshold ECG value, and a codefor the bin. A serial accumulator is set to a pre-determined value, suchas a center value of an ECG recorder. For each of theelectrocardiography values remaining in the series of theelectrocardiography values, a recursive process is performed thatincludes the following processes: selecting the electrocardiographyvalue next remaining in the series of the electrocardiography values;taking a difference of the selected electrocardiography value and theserial accumulator; identifying the bin in the plurality of the binscorresponding to the difference; representing the selectedelectrocardiography value by the code for the identified bin; andadjusting the serial accumulator by a value derived from the identifiedbin. The resulting string of represented codes are written into asequence in a non-volatile memory.

Another embodiment provides a computer-implemented method for encodingand compressing electrocardiography values. A series ofelectrocardiography values is obtained. A plurality of bins are defined,each bin comprising a lower threshold ECG value, an upper threshold ECGvalue, and a code for the bin. A serial accumulator is set to apre-determined value, such as a center value of an ECG recorder. Foreach of the electrocardiography values remaining in the series of theelectrocardiography values, a recursive process is performed thatincludes the following processes: selecting the electrocardiographyvalue next remaining in the series of electrocardiography values; takinga difference of the selected electrocardiography value and the serialaccumulator; identifying the bin in the plurality of the binscorresponding to the difference; representing the selectedelectrocardiography value by the code for the identified bin; andadjusting the serial accumulator by a value derived from the identifiedbin. The recursive process generates a sequence of represented codes.The sequence of represented codes is further encoded in the form of asingle number between 0 and 1, through the following steps: setting arange for an initial code from the sequence of the represented codes,processing each of the codes remaining in the sequence of therepresented codes, by a recursive process of: obtaining an estimation ofprobabilities of next codes; dividing the range into sub-ranges, eachsub-range representing a fraction of the range proportional to theprobabilities of the next codes; obtaining a next code; selecting thesub-range corresponding to the next code; representing the next code bythe selected sub-range; substituting the selected sub-range in place ofthe range and continuing the steps of the process using the selectedsub-range in place of the range. During the process, strings of codesrepresented by the selected sub-ranges are encoded into part of thesingle number between 0 and 1 and can be periodically or continuallystored into the non-volatile memory, can be stored on-demand oras-needed, or can be queued up and stored en masse upon completion ofthe process.

When only the first compression step is performed, unnecessary data inECG data are filtered out, and the number of codes needed to encode theECG data are reduced. When the second compression step is combined withthe first step, further memory consumption is avoided.

The foregoing aspects enhance ECG monitoring performance and quality byfacilitating long-term ECG recording, which is critical to accuratearrhythmia and cardiac rhythm disorder diagnoses.

The monitoring patch is especially suited to the female anatomy,although also easily used over the male sternum. The narrow longitudinalmidsection can fit nicely within the inter-mammary cleft of the breastswithout inducing discomfort, whereas conventional patch electrodes arewide and, if adhered between the breasts, would cause chafing,irritation, discomfort, and annoyance, leading to low patientcompliance.

In addition, the foregoing aspects enhance comfort in women (and certainmen), but not irritation of the breasts, by placing the monitoring patchin the best location possible for optimizing the recording of cardiacsignals from the atrium, particularly P-waves, which is another featurecritical to proper arrhythmia and cardiac rhythm disorder diagnoses.

Still other embodiments will become readily apparent to those skilled inthe art from the following detailed description, wherein are describedembodiments by way of illustrating the best mode contemplated. As willbe realized, other and different embodiments are possible and theembodiments' several details are capable of modifications in variousobvious respects, all without departing from their spirit and the scope.Accordingly, the drawings and detailed description are to be regarded asillustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are diagrams showing, by way of examples, an extended wearelectrocardiography monitor, including an extended wear electrode patch,in accordance with one embodiment, respectively fitted to the sternalregion of a female patient and a male patient.

FIG. 3 is a front anatomical view showing, by way of illustration, thelocations of the heart and lungs within the rib cage of an adult human.

FIG. 4 is a perspective view showing an extended wear electrode patch inaccordance with one embodiment with a monitor recorder inserted.

FIG. 5 is a perspective view showing the monitor recorder of FIG. 4.

FIG. 6 is a perspective view showing the extended wear electrode patchof FIG. 4 without a monitor recorder inserted.

FIG. 7 is a bottom plan view of the monitor recorder of FIG. 4.

FIG. 8 is a top view showing the flexible circuit of the extended wearelectrode patch of FIG. 4.

FIG. 9 is a functional block diagram showing the component architectureof the circuitry of the monitor recorder of FIG. 4.

FIG. 10 is a functional block diagram showing the circuitry of theextended wear electrode patch of FIG. 4.

FIG. 11 is a schematic diagram showing the ECG front end circuit of thecircuitry of the monitor recorder of FIG. 9.

FIG. 12 is a flow diagram showing a monitor recorder-implemented methodfor monitoring ECG data for use in the monitor recorder of FIG. 4.

FIG. 13 is a graph showing, by way of example, a typical ECG waveform.

FIG. 14 is a functional block diagram showing the signal processingfunctionality of the microcontroller.

FIG. 15 is a functional block diagram showing the operations performedby the download station.

FIGS. 16A-C are functional block diagrams respectively showing practicaluses of the extended wear electrocardiography monitors of FIGS. 1 and 2.

FIG. 17 is a perspective view of an extended wear electrode patch with aflexile wire electrode assembly in accordance with a still furtherembodiment.

FIG. 18 is perspective view of the flexile wire electrode assembly fromFIG. 17, with a layer of insulating material shielding a bare distalwire around the midsection of the flexible backing.

FIG. 19 is a bottom view of the flexile wire electrode assembly as shownin FIG. 17.

FIG. 20 is a bottom view of a flexile wire electrode assembly inaccordance with a still yet further embodiment.

FIG. 21 is a perspective view showing the longitudinal midsection of theflexible backing of the electrode assembly from FIG. 17.

FIG. 22 is a flow diagram showing a monitor recorder-implemented methodfor ECG signal processing and ECG data compressing for use in themonitor recorders of FIG. 4.

FIG. 23 is a flow diagram showing a monitor recorder-implemented methodfor encoding ECG values.

FIG. 24 is an example of a panel of codes or encodings with each codecovering a range defined by a lower threshold ECG value and an upperthreshold ECG value.

FIG. 25 is an illustrating the encoding and compression scheme inaccordance with method and parameters as described with reference to inFIGS. 23 and 24.

FIG. 26 is a flow diagram showing a monitor recorder-implemented methodfor further compressing the encodings.

DETAILED DESCRIPTION

ECG and physiological monitoring can be provided through a wearableambulatory monitor that includes two components, a flexible extendedwear electrode patch and a removable reusable (or single use) monitorrecorder. Both the electrode patch and the monitor recorder areoptimized to capture electrical signals from the propagation of lowamplitude, relatively low frequency content cardiac action potentials,particularly the P-waves generated during atrial activation. FIGS. 1 and2 are diagrams showing, by way of examples, an extended wearelectrocardiography monitor 12, including a monitor recorder 14, inaccordance with one embodiment, respectively fitted to the sternalregion of a female patient 10 and a male patient 11. The wearablemonitor 12 sits centrally, positioned axially along the sternal midline16, on the patient's chest along the sternum 13 and orientedtop-to-bottom with the monitor recorder 14 preferably situated towardsthe patient's head. In a further embodiment, the orientation of thewearable monitor 12 can be corrected post-monitoring, as furtherdescribed infra, for instance, if the wearable monitor 12 isinadvertently fitted upside down.

The electrode patch 15 is shaped to fit comfortably and conformal to thecontours of the patient's chest approximately centered on the sternalmidline 16 (or immediately to either side of the sternum 13). The distalend of the electrode patch 15, under which a lower or inferior pole (ECGelectrode) is adhered, extends towards the Xiphoid process and lowersternum and, depending upon the patient's build, may straddle the regionover the Xiphoid process and lower sternum. The proximal end of theelectrode patch 15, located under the monitor recorder 14, under whichan upper or superior pole (ECG electrode) is adhered, is below themanubrium and, depending upon patient's build, may straddle the regionover the manubrium.

During ECG monitoring, the amplitude and strength of action potentialssensed on the body's surface are affected to varying degrees by cardiac,cellular, extracellular, vector of current flow, and physical factors,like obesity, dermatitis, large breasts, and high impedance skin, as canoccur in dark-skinned individuals. Sensing along the sternal midline 16(or immediately to either side of the sternum 13) significantly improvesthe ability of the wearable monitor 12 to cutaneously sense cardiacelectric signals, particularly the P-wave (or atrial activity) and, to alesser extent, the QRS interval signals in the ECG waveforms thatindicate ventricular activity by countering some of the effects of thesefactors.

The ability to sense low amplitude, low frequency content body surfacepotentials is directly related to the location of ECG electrodes on theskin's surface and the ability of the sensing circuitry to capture theseelectrical signals. FIG. 3 is a front anatomical view showing, by way ofillustration, the locations of the heart 4 and lungs 5 within the ribcage of an adult human. Depending upon their placement locations on thechest, ECG electrodes may be separated from activation regions withinthe heart 4 by differing combinations of internal tissues and bodystructures, including heart muscle, intracardiac blood, the pericardium,intrathoracic blood and fluids, the lungs 5, skeletal muscle, bonestructure, subcutaneous fat, and the skin, plus any contaminants presentbetween the skin's surface and electrode signal pickups. The degree ofamplitude degradation of cardiac transmembrane potentials increases withthe number of tissue boundaries between the heart 4 and the skin'ssurface that are encountered. The cardiac electrical field is degradedeach time the transmembrane potentials encounter a physical boundaryseparating adjoining tissues due to differences in the respectivetissues' electrical resistances. In addition, other non-spatial factors,such as pericardial effusion, emphysema or fluid accumulation in thelungs, as further explained infra, can further degrade body surfacepotentials.

Internal tissues and body structures can adversely affect the currentstrength and signal fidelity of all body surface potentials, yet lowamplitude cardiac action potentials, particularly the P-wave with anormative amplitude of less than 0.25 microvolts (mV) and a normativeduration of less than 120 milliseconds (ms), are most apt to benegatively impacted. The atria 6 are generally located posteriorlywithin the thoracic cavity (with the exception of the anterior rightatrium and right atrial appendage), and, physically, the left atriumconstitutes the portion of the heart 4 furthest away from the surface ofthe skin on the chest. Conversely, the ventricles 7, which generatelarger amplitude signals, generally are located anteriorly with theanterior right ventricle and most of the left ventricle situatedrelatively close to the skin surface on the chest, which contributes tothe relatively stronger amplitudes of ventricular waveforms. Thus, thequality of P-waves (and other already-low amplitude action potentialsignals) is more susceptible to weakening from intervening tissues andstructures than the waveforms associated with ventricular activation.

The importance of the positioning of ECG electrodes along the sternalmidline 15 has largely been overlooked by conventional approaches to ECGmonitoring, in part due to the inability of their sensing circuitry toreliably detect low amplitude, low frequency content electrical signals,particularly in P-waves. In turn, that inability to keenly sense P-waveshas motivated ECG electrode placement in other non-sternal midlinethoracic locations, where the QRSTU components that representventricular electrical activity are more readily detectable by theirsensing circuitry than P-waves. In addition, ECG electrode placementalong the sternal midline 15 presents major patient wearabilitychallenges, such as fitting a monitoring ensemble within the narrowconfines of the inter-mammary cleft between the breasts, that to largeextent drive physical packaging concerns, which can be incompatible withECG monitors intended for placement, say, in the upper pectoral regionor other non-sternal midline thoracic locations. In contrast, thewearable monitor 12 uses an electrode patch 15 that is specificallyintended for extended wear placement in a location at the sternalmidline 16 (or immediately to either side of the sternum 13). Whencombined with a monitor recorder 14 that uses sensing circuitryoptimized to preserve the characteristics of low amplitude cardiacaction potentials, especially those signals from the atria, as furtherdescribed infra with reference to FIG. 11, the electrode patch 15 helpsto significantly improve atrial activation (P-wave) sensing throughplacement in a body location that robustly minimizes the effects oftissue and body structure.

Referring back to FIGS. 1 and 2, the placement of the wearable monitor12 in the region of the sternal midline 13 puts the ECG electrodes ofthe electrode patch 15 in locations better adapted to sensing andrecording low amplitude cardiac action potentials during atrialpropagation (P-wave signals) than placement in other locations, such asthe upper left pectoral region, as commonly seen in most conventionalambulatory ECG monitors. The sternum 13 overlies the right atrium of theheart 4. As a result, action potential signals have to travel throughfewer layers of tissue and structure to reach the ECG electrodes of theelectrode patch 15 on the body's surface along the sternal midline 13when compared to other monitoring locations, a distinction that is ofcritical importance when capturing low frequency content electricalsignals, such as P-waves.

Moreover, cardiac action potential propagation travels simultaneouslyalong a north-to-south and right-to-left vector, beginning high in theright atrium and ultimately ending in the posterior and lateral regionof the left ventricle. Cardiac depolarization originates high in theright atrium in the SA node before concurrently spreading leftwardtowards the left atrium and inferiorly towards the AV node. The ECGelectrodes of the electrode patch 15 are placed with the upper orsuperior pole (ECG electrode) along the sternal midline 13 in the regionof the manubrium and the lower or inferior pole (ECG electrode) alongthe sternal midline 13 in the region of the Xiphoid process 9 and lowersternum. The ECG electrodes are placed primarily in a north-to-southorientation along the sternum 13 that corresponds to the north-to-southwaveform vector exhibited during atrial activation. This orientationcorresponds to the aVF lead used in a conventional 12-lead ECG that isused to sense positive or upright P-waves.

Furthermore, the thoracic region underlying the sternum 13 along themidline 16 between the manubrium 8 and Xiphoid process 9 is relativelyfree of lung tissue, musculature, and other internal body structuresthat could occlude the electrical signal path between the heart 4,particularly the atria, and ECG electrodes placed on the surface of theskin. Fewer obstructions means that cardiac electrical potentialsencounter fewer boundaries between different tissues. As a result, whencompared to other thoracic ECG sensing locations, the cardiac electricalfield is less altered when sensed dermally along the sternal midline 13.As well, the proximity of the sternal midline 16 to the ventricles 7facilitates sensing of right ventricular activity and provides superiorrecordation of the QRS interval, again, in part due to the relativelyclear electrical path between the heart 4 and the skin surface.

Finally, non-spatial factors can affect transmembrane action potentialshape and conductivity. For instance, myocardial ischemia, an acutecardiac condition, can cause a transient increase in blood perfusion inthe lungs 5. The perfused blood can significantly increase electricalresistance across the lungs 5 and therefore degrade transmission of thecardiac electrical field to the skin's surface. However, the placementof the wearable monitor 12 along the sternal midline 16 in theinter-mammary cleft between the breasts is relatively resilient to theadverse effects to cardiac action potential degradation caused byischemic conditions as the body surface potentials from a locationrelatively clear of underlying lung tissue and fat help compensate forthe loss of signal amplitude and content. The monitor recorder 14 isthus able to record the P-wave morphology that may be compromised bymyocardial ischemia and therefore make diagnosis of the specificarrhythmias that can be associated with myocardial ischemia moredifficult.

During use, the electrode patch 15 is first adhered to the skin alongthe sternal midline 16 (or immediately to either side of the sternum13). A monitor recorder 14 is then snapped into place on the electrodepatch 15 using an electro mechanical docking interface to initiate ECGmonitoring. FIG. 4 is a perspective view showing an extended wearelectrode patch 15 in accordance with one embodiment with a monitorrecorder 14 inserted. The body of the electrode patch 15 is preferablyconstructed using a flexible backing 20 formed as an elongated strip 21of wrap knit or similar stretchable material about 145 mm long and 32 mmat the widest point with a narrow longitudinal mid-section 23 evenlytapering inward from both sides. A pair of cut-outs 22 between thedistal and proximal ends of the electrode patch 15 create a narrowlongitudinal midsection 23 or “isthmus” and defines an elongated“hourglass”-like shape, when viewed from above, such as described incommonly-assigned U.S. Design Patent application, entitled “ExtendedWear Electrode Patch,” Ser. No. 29/472,045, filed Nov. 7, 2013, pending,the disclosure of which is incorporated by reference. The upper part ofthe “hourglass” is sized to allow an electrically non-conductivereceptacle 25, sits on top of the outward-facing surface of theelectrode patch 15, to be affixed to the electrode patch 15 with an ECGelectrode placed underneath on the patient-facing underside, or contact,surface of the electrode patch 15; the upper part of the “hourglass” hasa longer and wider profile (but still rounded and tapered to fitcomfortably between the breasts) than the lower part of the “hourglass,”which is sized primarily to allow just the placement of an ECG electrodeof appropriate shape and surface area to record the P-wave and the QRSsignals sufficiently given the inter-electrode spacing.

The electrode patch 15 incorporates features that significantly improvewearability, performance, and patient comfort throughout an extendedmonitoring period. The entire electrode patch 15 is lightweight inconstruction, which allows the patch to be resilient to disadhesing orfalling off and, critically, to avoid creating distracting discomfort tothe patient, even when the patient is asleep. In contrast, the weight ofa heavy ECG monitor impedes patient mobility and will cause the monitorto constantly tug downwards and press on the patient's body that cangenerate skin inflammation with frequent adjustments by the patientneeded to maintain comfort.

During everyday wear, the electrode patch 15 is subjected to pushing,pulling, and torsional movements, including compressional and torsionalforces when the patient bends forward, or tensile and torsional forceswhen the patient leans backwards. To counter these stress forces, theelectrode patch 15 incorporates crimp and strain reliefs, such asdescribed in commonly-assigned U.S. Patent application, entitled“Extended Wear Electrocardiography Patch,” Ser. No. 14/080,717, filedNov. 14, 2013, pending, the disclosure of which is incorporated byreference. In addition, the cut-outs 22 and longitudinal midsection 23help minimize interference with and discomfort to breast tissue,particularly in women (and gynecomastic men). The cut-outs 22 andlongitudinal midsection 23 further allow better conformity of theelectrode patch 15 to sternal bowing and to the narrow isthmus of flatskin that can occur along the bottom of the inter-mammary cleft betweenthe breasts, especially in buxom women. The cut-outs 22 and narrow andflexible longitudinal midsection 23 help the electrode patch 15 fitnicely between a pair of female breasts in the inter-mammary cleft. Inone embodiment, the cut-outs 22 can be graduated to form thelongitudinal midsection 23 as a narrow in-between stem or isthmusportion about 7 mm wide. In a still further embodiment, tabs 24 canrespectively extend an additional 8 mm to 12 mm beyond the distal andproximal ends of the flexible backing 20 to facilitate with adhering theelectrode patch 15 to or removing the electrode patch 15 from thesternum 13. These tabs preferably lack adhesive on the underside, orcontact, surface of the electrode patch 15. Still other shapes, cut-outsand conformities to the electrode patch 15 are possible.

The monitor recorder 14 removably and reusably snaps into anelectrically non-conductive receptacle 25 during use. The monitorrecorder 14 contains electronic circuitry for recording and storing thepatient's electrocardiography as sensed via a pair of ECG electrodesprovided on the electrode patch 15, as further described infra beginningwith reference to FIG. 9. The non-conductive receptacle 25 is providedon the top surface of the flexible backing 20 with a retention catch 26and tension clip 27 molded into the non-conductive receptacle 25 toconformably receive and securely hold the monitor recorder 14 in place.

The monitor recorder 14 includes a sealed housing that snaps into placein the non-conductive receptacle 25. FIG. 5 is a perspective viewshowing the monitor recorder 14 of FIG. 4. The sealed housing 50 of themonitor recorder 14 intentionally has a rounded isoscelestrapezoidal-like shape 52, when viewed from above, such as described incommonly-assigned U.S. Design Patent application, entitled“Electrocardiography Monitor,” Ser. No. 29/472,046, filed Nov. 7, 2013,pending, the disclosure of which is incorporated by reference. The edges51 along the top and bottom surfaces are rounded for patient comfort.The sealed housing 50 is approximately 47 mm long, 23 mm wide at thewidest point, and 7 mm high, excluding a patient-operabletactile-feedback button 55. The sealed housing 50 can be molded out ofpolycarbonate, ABS, or an alloy of those two materials. The button 55 iswaterproof and the button's top outer surface is molded silicon rubberor similar soft pliable material. A retention detent 53 and tensiondetent 54 are molded along the edges of the top surface of the housing50 to respectively engage the retention catch 26 and the tension clip 27molded into non-conductive receptacle 25. Other shapes, features, andconformities of the sealed housing 50 are possible.

The electrode patch 15 is intended to be disposable, while the monitorrecorder 14 is designed for reuse and can be transferred to successiveelectrode patches 15 to ensure continuity of monitoring, if so desired.The monitor recorder 14 can be used only once, but single useeffectively wastes the synergistic benefits provided by the combinationof the disposable electrode patch and reusable monitor recorder, asfurther explained infra with reference to FIGS. 16A-C. The placement ofthe wearable monitor 12 in a location at the sternal midline 16 (orimmediately to either side of the sternum 13) benefits long-termextended wear by removing the requirement that ECG electrodes becontinually placed in the same spots on the skin throughout themonitoring period. Instead, the patient is free to place an electrodepatch 15 anywhere within the general region of the sternum 13.

As a result, at any point during ECG monitoring, the patient's skin isable to recover from the wearing of an electrode patch 15, whichincreases patient comfort and satisfaction, while the monitor recorder14 ensures ECG monitoring continuity with minimal effort. A monitorrecorder 14 is merely unsnapped from a worn out electrode patch 15, theworn out electrode patch 15 is removed from the skin, a new electrodepatch 15 is adhered to the skin, possibly in a new spot immediatelyadjacent to the earlier location, and the same monitor recorder 14 issnapped into the new electrode patch 15 to reinitiate and continue theECG monitoring.

During use, the electrode patch 15 is first adhered to the skin in thesternal region. FIG. 6 is a perspective view showing the extended wearelectrode patch 15 of FIG. 4 without a monitor recorder 14 inserted. Aflexible circuit 32 is adhered to each end of the flexible backing 20. Adistal circuit trace 33 from the distal end 30 of the flexible backing20 and a proximal circuit trace (not shown) from the proximal end 31 ofthe flexible backing 20 electrically couple ECG electrodes (not shown)with a pair of electrical pads 34. In a further embodiment, the distaland proximal circuit traces are replaced with interlaced or sewn-inflexible wires, as further described infra beginning with reference toFIG. 17. The electrical pads 34 are provided within a moisture-resistantseal 35 formed on the bottom surface of the non-conductive receptacle25. When the monitor recorder 14 is securely received into thenon-conductive receptacle 25, that is, snapped into place, theelectrical pads 34 interface to electrical contacts (not shown)protruding from the bottom surface of the monitor recorder 14. Themoisture-resistant seal 35 enables the monitor recorder 14 to be worn atall times, even during showering or other activities that could exposethe monitor recorder 14 to moisture or adverse conditions.

In addition, a battery compartment 36 is formed on the bottom surface ofthe non-conductive receptacle 25. A pair of battery leads (not shown)from the battery compartment 36 to another pair of the electrical pads34 electrically interface the battery to the monitor recorder 14. Thebattery contained within the battery compartment 35 is a direct current(DC) power cell and can be replaceable, rechargeable or disposable.

The monitor recorder 14 draws power externally from the battery providedin the non-conductive receptacle 25, thereby uniquely obviating the needfor the monitor recorder 14 to carry a dedicated power source. FIG. 7 isa bottom plan view of the monitor recorder 14 of FIG. 4. A cavity 58 isformed on the bottom surface of the sealed housing 50 to accommodate theupward projection of the battery compartment 36 from the bottom surfaceof the non-conductive receptacle 25, when the monitor recorder 14 issecured in place on the non-conductive receptacle 25. A set ofelectrical contacts 56 protrude from the bottom surface of the sealedhousing 50 and are arranged in alignment with the electrical pads 34provided on the bottom surface of the non-conductive receptacle 25 toestablish electrical connections between the electrode patch 15 and themonitor recorder 14. In addition, a seal coupling 57 circumferentiallysurrounds the set of electrical contacts 56 and securely mates with themoisture-resistant seal 35 formed on the bottom surface of thenon-conductive receptacle 25. The battery contained within the batterycompartment 36 can be replaceable, rechargeable or disposable. In afurther embodiment, the ECG sensing circuitry of the monitor recorder 14can be supplemented with additional sensors, including an SpO₂ sensor, ablood pressure sensor, a temperature sensor, respiratory rate sensor, aglucose sensor, an air flow sensor, and a volumetric pressure sensor,which can be incorporated directly into the monitor recorder 14 or ontothe non-conductive receptacle 25.

The placement of the flexible backing 20 on the sternal midline 16 (orimmediately to either side of the sternum 13) also helps to minimize theside-to-side movement of the wearable monitor 12 in the left- andright-handed directions during wear. However, the wearable monitor 12 isstill susceptible to pushing, pulling, and torqueing movements,including compressional and torsional forces when the patient bendsforward, and tensile and torsional forces when the patient leansbackwards or twists. To counter the dislodgment of the flexible backing20 due to compressional and torsional forces, a layer of non-irritatingadhesive, such as hydrocolloid, is provided at least partially on theunderside, or contact, surface of the flexible backing 20, but only onthe distal end 30 and the proximal end 31. As a result, the underside,or contact surface of the longitudinal midsection 23 does not have anadhesive layer and remains free to move relative to the skin. Thus, thelongitudinal midsection 23 forms a crimp relief that respectivelyfacilitates compression and twisting of the flexible backing 20 inresponse to compressional and torsional forces. Other forms of flexiblebacking crimp reliefs are possible.

Unlike the flexible backing 20, the flexible circuit 32 is only able tobend and cannot stretch in a planar direction. The flexible circuit 32can be provided either above or below the flexible backing 20. FIG. 8 isa top view showing the flexible circuit 32 of the extended wearelectrode patch 15 of FIG. 4 when mounted above the flexible backing 20.A distal ECG electrode 38 and proximal ECG electrode 39 are respectivelycoupled to the distal and proximal ends of the flexible circuit 32 toserve as electrode signal pickups. The flexible circuit 32 preferablydoes not extend to the outside edges of the flexible backing 20, therebyavoiding gouging or discomforting the patient's skin during extendedwear, such as when sleeping on the side. During wear, the ECG electrodes38, 39 must remain in continual contact with the skin. A strain relief40 is defined in the flexible circuit 32 at a location that is partiallyunderneath the battery compartment 36 when the flexible circuit 32 isaffixed to the flexible backing 20. The strain relief 40 is laterallyextendable to counter dislodgment of the ECG electrodes 38, 39 due tobending, tensile and torsional forces. A pair of strain relief cutouts41 partially extend transversely from each opposite side of the flexiblecircuit 32 and continue longitudinally towards each other to define in‘S’-shaped pattern, when viewed from above. The strain reliefrespectively facilitates longitudinal extension and twisting of theflexible circuit 32 in response to tensile and torsional forces. Otherforms of circuit board strain relief are possible.

ECG monitoring and other functions performed by the monitor recorder 14are provided through a micro controlled architecture. FIG. 9 is afunctional block diagram showing the component architecture of thecircuitry 60 of the monitor recorder 14 of FIG. 4. The circuitry 60 isexternally powered through a battery provided in the non-conductivereceptacle 25 (shown in FIG. 6). Both power and raw ECG signals, whichoriginate in the pair of ECG electrodes 38, 39 (shown in FIG. 8) on thedistal and proximal ends of the electrode patch 15, are received throughan external connector 65 that mates with a corresponding physicalconnector on the electrode patch 15. The external connector 65 includesthe set of electrical contacts 56 that protrude from the bottom surfaceof the sealed housing 50 and which physically and electrically interfacewith the set of pads 34 provided on the bottom surface of thenon-conductive receptacle 25. The external connector includes electricalcontacts 56 for data download, microcontroller communications, power,analog inputs, and a peripheral expansion port. The arrangement of thepins on the electrical connector 65 of the monitor recorder 14 and thedevice into which the monitor recorder 14 is attached, whether anelectrode patch 15 or download station (not shown), follow the sameelectrical pin assignment convention to facilitate interoperability. Theexternal connector 65 also serves as a physical interface to a downloadstation that permits the retrieval of stored ECG monitoring data,communication with the monitor recorder 14, and performance of otherfunctions. The download station is further described infra withreference to FIG. 15.

Operation of the circuitry 60 of the monitor recorder 14 is managed by amicrocontroller 61, such as the EFM32 Tiny Gecko 32-bit microcontroller,manufactured by Silicon Laboratories Inc., Austin, Tex. Themicrocontroller 61 has flexible energy management modes and includes adirect memory access controller and built-in analog-to-digital anddigital-to-analog converters (ADC and DAC, respectively). Themicrocontroller 61 also includes a program memory unit containinginternal flash memory that is readable and writeable. The internal flashmemory can also be programmed externally. The microcontroller 61operates under modular micro program control as specified in firmwarestored in the internal flash memory. The functionality and firmwaremodules relating to signal processing by the microcontroller 61 arefurther described infra with reference to FIG. 14. The microcontroller61 draws power externally from the battery provided on the electrodepatch 15 via a pair of the electrical contacts 56. The microcontroller61 connects to the ECG front end circuit 63 that measures raw cutaneouselectrical signals using a driven reference that eliminates common modenoise, as further described infra with reference to FIG. 11.

The circuitry 60 of the monitor recorder 14 also includes a flash memory62, which the microcontroller 61 uses for storing ECG monitoring dataand other physiology and information. The flash memory 62 also drawspower externally from the battery provided on the electrode patch 15 viaa pair of the electrical contacts 56. Data is stored in a serial flashmemory circuit, which supports read, erase and program operations over acommunications bus. The flash memory 62 enables the microcontroller 61to store digitized ECG data. The communications bus further enables theflash memory 62 to be directly accessed externally over the externalconnector 65 when the monitor recorder 14 is interfaced to a downloadstation.

The microcontroller 61 includes functionality that enables theacquisition of samples of analog ECG signals, which are converted into adigital representation, as further described infra with reference toFIG. 14. In one mode, the microcontroller 61 will acquire, sample,digitize, signal process, and store digitized ECG data into availablestorage locations in the flash memory 62 until all memory storagelocations are filled, after which the digitized ECG data needs to bedownloaded or erased to restore memory capacity. Data download orerasure can also occur before all storage locations are filled, whichwould free up memory space sooner, albeit at the cost of possiblyinterrupting monitoring while downloading or erasure is performed. Inanother mode, the microcontroller 61 can include a loop recorder featurethat will overwrite the oldest stored data once all storage locationsare filled, albeit at the cost of potentially losing the stored datathat was overwritten, if not previously downloaded. Still other modes ofdata storage and capacity recovery are possible.

The circuitry 60 of the monitor recorder 14 further includes anactigraphy sensor 64 implemented as a 3-axis accelerometer. Theaccelerometer may be configured to generate interrupt signals to themicrocontroller 61 by independent initial wake up and free fall events,as well as by device position. In addition, the actigraphy provided bythe accelerometer can be used during post-monitoring analysis to correctthe orientation of the monitor recorder 14 if, for instance, the monitorrecorder 14 has been inadvertently installed upside down, that is, withthe monitor recorder 14 oriented on the electrode patch 15 towards thepatient's feet, as well as for other event occurrence analyses.

The microcontroller 61 includes an expansion port that also utilizes thecommunications bus. External devices, separately drawing powerexternally from the battery provided on the electrode patch 15 or othersource, can interface to the microcontroller 61 over the expansion portin half duplex mode. For instance, an external physiology sensor can beprovided as part of the circuitry 60 of the monitor recorder 14, or canbe provided on the electrode patch 15 with communication with themicrocontroller 61 provided over one of the electrical contacts 56. Thephysiology sensor can include an SpO₂ sensor, blood pressure sensor,temperature sensor, respiratory rate sensor, glucose sensor, airflowsensor, volumetric pressure sensing, or other types of sensor ortelemetric input sources. In a further embodiment, a wireless interfacefor interfacing with other wearable (or implantable) physiologymonitors, as well as data offload and programming, can be provided aspart of the circuitry 60 of the monitor recorder 14, or can be providedon the electrode patch 15 with communication with the microcontroller 61provided over one of the electrical contacts 56.

Finally, the circuitry 60 of the monitor recorder 14 includespatient-interfaceable components, including a tactile feedback button66, which a patient can press to mark events or to perform otherfunctions, and a buzzer 67, such as a speaker, magnetic resonator orpiezoelectric buzzer. The buzzer 67 can be used by the microcontroller61 to output feedback to a patient such as to confirm power up andinitiation of ECG monitoring. Still other components as part of thecircuitry 60 of the monitor recorder 14 are possible.

While the monitor recorder 14 operates under micro control, most of theelectrical components of the electrode patch 15 operate passively. FIG.10 is a functional block diagram showing the circuitry 70 of theextended wear electrode patch 15 of FIG. 4. The circuitry 70 of theelectrode patch 15 is electrically coupled with the circuitry 60 of themonitor recorder 14 through an external connector 74. The externalconnector 74 is terminated through the set of pads 34 provided on thebottom of the non-conductive receptacle 25, which electrically mate tocorresponding electrical contacts 56 protruding from the bottom surfaceof the sealed housing 50 to electrically interface the monitor recorder14 to the electrode patch 15.

The circuitry 70 of the electrode patch 15 performs three primaryfunctions. First, a battery 71 is provided in a battery compartmentformed on the bottom surface of the non-conductive receptacle 25. Thebattery 71 is electrically interfaced to the circuitry 60 of the monitorrecorder 14 as a source of external power. The unique provisioning ofthe battery 71 on the electrode patch 15 provides several advantages.First, the locating of the battery 71 physically on the electrode patch15 lowers the center of gravity of the overall wearable monitor 12 andthereby helps to minimize shear forces and the effects of movements ofthe patient and clothing. Moreover, the housing 50 of the monitorrecorder 14 is sealed against moisture and providing power externallyavoids having to either periodically open the housing 50 for the batteryreplacement, which also creates the potential for moisture intrusion andhuman error, or to recharge the battery, which can potentially take themonitor recorder 14 off line for hours at a time. In addition, theelectrode patch 15 is intended to be disposable, while the monitorrecorder 14 is a reusable component. Each time that the electrode patch15 is replaced, a fresh battery is provided for the use of the monitorrecorder 14, which enhances ECG monitoring performance quality andduration of use. Also, the architecture of the monitor recorder 14 isopen, in that other physiology sensors or components can be added byvirtue of the expansion port of the microcontroller 61. Requiring thoseadditional sensors or components to draw power from a source external tothe monitor recorder 14 keeps power considerations independent of themonitor recorder 14. This approach also enables a battery of highercapacity to be introduced when needed to support the additional sensorsor components without effecting the monitor recorders circuitry 60.

Second, the pair of ECG electrodes 38, 39 respectively provided on thedistal and proximal ends of the flexible circuit 32 are electricallycoupled to the set of pads 34 provided on the bottom of thenon-conductive receptacle 25 by way of their respective circuit traces33, 37. The signal ECG electrode 39 includes a protection circuit 72,which is an inline resistor that protects the patient from excessiveleakage current should the front end circuit fail.

Last, in a further embodiment, the circuitry 70 of the electrode patch15 includes a cryptographic circuit 73 to authenticate an electrodepatch 15 for use with a monitor recorder 14. The cryptographic circuit73 includes a device capable of secure authentication and validation.The cryptographic device 73 ensures that only genuine, non-expired,safe, and authenticated electrode patches 15 are permitted to providemonitoring data to a monitor recorder 14 and for a specific patient.

The ECG front end circuit 63 measures raw cutaneous electrical signalsusing a driven reference that effectively reduces common mode noise,power supply noise and system noise, which is critical to preserving thecharacteristics of low amplitude cardiac action potentials, especiallythose signals from the atria. FIG. 11 is a schematic diagram 80 showingthe ECG front end circuit 63 of the circuitry 60 of the monitor recorder14 of FIG. 9. The ECG front end circuit 63 senses body surfacepotentials through a signal lead (“S1”) and reference lead (“REF”) thatare respectively connected to the ECG electrodes of the electrode patch15. Power is provided to the ECG front end circuit 63 through a pair ofDC power leads (“VCC” and “GND”). An analog ECG signal (“ECG”)representative of the electrical activity of the patient's heart overtime is output, which the micro controller 11 converts to digitalrepresentation and filters, as further described infra.

The ECG front end circuit 63 is organized into five stages, a passiveinput filter stage 81, a unity gain voltage follower stage 82, a passivehigh pass filtering stage 83, a voltage amplification and activefiltering stage 84, and an anti-aliasing passive filter stage 85, plus areference generator. Each of these stages and the reference generatorwill now be described.

The passive input filter stage 81 includes the parasitic impedance ofthe ECG electrodes 38, 39 (shown in FIG. 8), the protection resistorthat is included as part of the protection circuit 72 of the ECGelectrode 39 (shown in FIG. 10), an AC coupling capacitor 87, atermination resistor 88, and filter capacitor 89. This stage passivelyshifts the frequency response poles downward there is a high electrodeimpedance from the patient on the signal lead S1 and reference lead REF,which reduces high frequency noise.

The unity gain voltage follower stage 82 provides a unity voltage gainthat allows current amplification by an Operational Amplifier (“Op Amp”)90. In this stage, the voltage stays the same as the input, but morecurrent is available to feed additional stages. This configurationallows a very high input impedance, so as not to disrupt the bodysurface potentials or the filtering effect of the previous stage.

The passive high pass filtering stage 83 is a high pass filter thatremoves baseline wander and any offset generated from the previousstage. Adding an AC coupling capacitor 91 after the Op Amp 90 allows theuse of lower cost components, while increasing signal fidelity.

The voltage amplification and active filtering stage 84 amplifies thevoltage of the input signal through Op Amp 91, while applying a low passfilter. The DC bias of the input signal is automatically centered in thehighest performance input region of the Op Amp 91 because of the ACcoupling capacitor 91.

The anti-aliasing passive filter stage 85 provides an anti-aliasing lowpass filter. When the microcontroller 61 acquires a sample of the analoginput signal, a disruption in the signal occurs as a sample and holdcapacitor that is internal to the microcontroller 61 is charged tosupply signal for acquisition.

The reference generator in subcircuit 86 drives a driven referencecontaining power supply noise and system noise to the reference leadREF. A coupling capacitor 87 is included on the signal lead S1 and apair of resistors 93 a, 93 b inject system noise into the reference leadREF. The reference generator is connected directly to the patient,thereby avoiding the thermal noise of the protection resistor that isincluded as part of the protection circuit 72.

In contrast, conventional ECG lead configurations try to balance signaland reference lead connections. The conventional approach suffers fromthe introduction of differential thermal noise, lower input common moderejection, increased power supply noise, increased system noise, anddifferential voltages between the patient reference and the referenceused on the device that can obscure, at times, extremely, low amplitudebody surface potentials.

Here, the parasitic impedance of the ECG electrodes 38, 39, theprotection resistor that is included as part of the protection circuit72 and the coupling capacitor 87 allow the reference lead REF to beconnected directly to the skin's surface without any further components.As a result, the differential thermal noise problem caused by pairingprotection resistors to signal and reference leads, as used inconventional approaches, is avoided.

The monitor recorder 14 continuously monitors the patient's heart rateand physiology. FIG. 12 is a flow diagram showing a monitorrecorder-implemented method 100 for monitoring ECG data for use in themonitor recorder 14 of FIG. 4. Initially, upon being connected to theset of pads 34 provided with the non-conductive receptacle 25 when themonitor recorder 14 is snapped into place, the microcontroller 61executes a power up sequence (step 101). During the power up sequence,the voltage of the battery 71 is checked, the state of the flash memory62 is confirmed, both in terms of operability check and availablecapacity, and microcontroller operation is diagnostically confirmed. Ina further embodiment, an authentication procedure between themicrocontroller 61 and the electrode patch 15 are also performed.

Following satisfactory completion of the power up sequence, an iterativeprocessing loop (steps 102-110) is continually executed by themicrocontroller 61. During each iteration (step 102) of the processingloop, the ECG frontend 63 (shown in FIG. 9) continually senses thecutaneous ECG electrical signals (step 103) via the ECG electrodes 38,29 and is optimized to maintain the integrity of the P-wave. A sample ofthe ECG signal is read (step 104) by the microcontroller 61 by samplingthe analog ECG signal that is output by the ECG front end circuit 63.FIG. 13 is a graph showing, by way of example, a typical ECG waveform120. The x-axis represents time in approximate units of tenths of asecond. The y-axis represents cutaneous electrical signal strength inapproximate units of millivolts. The P-wave 121 has a smooth, normallyupward, that is, positive, waveform that indicates atrialdepolarization. The QRS complex often begins with the downwarddeflection of a Q-wave 122, followed by a larger upward deflection of anR-wave 123, and terminated with a downward waveform of the S-wave 124,collectively representative of ventricular depolarization. The T-wave125 is normally a modest upward waveform, representative of ventriculardepolarization, while the U-wave 126, often not directly observable,indicates the recovery period of the Purkinje conduction fibers.

Sampling of the R-to-R interval enables heart rate informationderivation. For instance, the R-to-R interval represents the ventricularrate and rhythm, while the P-to-P interval represents the atrial rateand rhythm. Importantly, the PR interval is indicative ofatrioventricular (AV) conduction time and abnormalities in the PRinterval can reveal underlying heart disorders, thus representinganother reason why the P-wave quality achievable by the ambulatoryelectrocardiography monitoring patch optimized for capturing lowamplitude cardiac action potential propagation described herein ismedically unique and important. The long-term observation of these ECGindicia, as provided through extended wear of the wearable monitor 12,provides valuable insights to the patient's cardiac function symptoms,and overall well-being.

Referring back to FIG. 12, each sampled ECG signal, in quantized anddigitized form, is processed by signal processing modules as specifiedin firmware (step 105), as described infra, and temporarily staged in abuffer (step 106), pending compression preparatory to storage in theflash memory 62 (step 107). Following compression, the compressed ECGdigitized sample is again buffered (step 108), then written to the flashmemory 62 (step 109) using the communications bus. Processing continues(step 110), so long as the monitoring recorder 14 remains connected tothe electrode patch 15 (and storage space remains available in the flashmemory 62), after which the processing loop is exited (step 110) andexecution terminates. Still other operations and steps are possible.

The microcontroller 61 operates under modular micro program control asspecified in firmware, and the program control includes processing ofthe analog ECG signal output by the ECG front end circuit 63. FIG. 14 isa functional block diagram showing the signal processing functionality130 of the microcontroller 61. The microcontroller 61 operates undermodular micro program control as specified in firmware 132. The firmwaremodules 132 include high and low pass filtering 133, and compression134. Other modules are possible. The microcontroller 61 has a built-inADC, although ADC functionality could also be provided in the firmware132.

The ECG front end circuit 63 first outputs an analog ECG signal, whichthe ADC 131 acquires, samples and converts into an uncompressed digitalrepresentation. The microcontroller 61 includes one or more firmwaremodules 133 that perform filtering. In one embodiment, three low passfilters and two high pass filters are used. Following filtering, thedigital representation of the cardiac activation wave front amplitudesare compressed by a compression module 134 before being written out tostorage 135, as further described infra beginning with reference to FIG.22.

The download station executes a communications or offload program(“Offload”) or similar program that interacts with the monitor recorder14 via the external connector 65 to retrieve the stored ECG monitoringdata. FIG. 15 is a functional block diagram showing the operations 140performed by the download station. The download station could be aserver, personal computer, tablet or handheld computer, smart mobiledevice, or purpose-built programmer designed specific to the task ofinterfacing with a monitor recorder 14. Still other forms of downloadstation are possible, including download stations connected throughwireless interfacing using, for instance, a smart phone connected to themonitor recorder 14 through Bluetooth or Wi-Fi.

The download station is responsible for offloading stored ECG monitoringdata from a monitor recorder 14 and includes an electro mechanicaldocking interface by which the monitor recorder 14 is connected at theexternal connector 65. The download station operates under programmablecontrol as specified in software 141. The stored ECG monitoring dataretrieved from storage 142 on a monitor recorder 14 is firstdecompressed by a decompression module 143, which converts the storedECG monitoring data back into an uncompressed digital representationmore suited to signal processing than a compressed signal. The retrievedECG monitoring data may be stored into local storage for archivalpurposes, either in original compressed form, or as uncompressed.

The download station can include an array of filtering modules. Forinstance, a set of phase distortion filtering tools 144 may be provided,where corresponding software filters can be provided for each filterimplemented in the firmware executed by the microcontroller 61. Thedigital signals are run through the software filters in a reversedirection to remove phase distortion. For instance, a 45 Hertz high passfilter in firmware may have a matching reverse 45 Hertz high pass filterin software. Most of the phase distortion is corrected, that is,canceled to eliminate noise at the set frequency, but data at otherfrequencies in the waveform remain unaltered. As well, bidirectionalimpulse infinite response (IIR) high pass filters and reverse direction(symmetric) IIR low pass filters can be provided. Data is run throughthese filters first in a forward direction, then in a reverse direction,which generates a square of the response and cancels out any phasedistortion. This type of signal processing is particularly helpful withimproving the display of the ST-segment by removing low frequency noise.

An automatic gain control (AGC) module 145 can also be provided toadjust the digital signals to a usable level based on peak or averagesignal level or other metric. AGC is particularly critical tosingle-lead ECG monitors, where physical factors, such as the tilt ofthe heart, can affect the electrical field generated. On three-leadHolter monitors, the leads are oriented in vertical, horizontal anddiagonal directions. As a result, the horizontal and diagonal leads maybe higher amplitude and ECG interpretation will be based on one or bothof the higher amplitude leads. In contrast, the electrocardiographymonitor 12 has only a single lead that is oriented in the verticaldirection, so variations in amplitude will be wider than available withmulti-lead monitors, which have alternate leads to fall back upon.

In addition, AGC may be necessary to maintain compatibility withexisting ECG interpretation software, which is typically calibrated formulti-lead ECG monitors for viewing signals over a narrow range ofamplitudes. Through the AGC module 145, the gain of signals recorded bythe monitor recorder 14 of the electrocardiography monitor 12 can beattenuated up (or down) to work with FDA-approved commercially availableECG interpretation.

AGC can be implemented in a fixed fashion that is uniformly applied toall signals in an ECG recording, adjusted as appropriate on arecording-by-recording basis. Typically, a fixed AGC value is calculatedbased on how an ECG recording is received to preserve the amplituderelationship between the signals. Alternatively, AGC can be varieddynamically throughout an ECG recording, where signals in differentsegments of an ECG recording are amplified up (or down) by differingamounts of gain.

Typically, the monitor recorder 14 will record a high resolution, lowfrequency signal for the P-wave segment. However, for some patients, theresult may still be a visually small signal. Although high resolution ispresent, the unaided eye will normally be unable to discern the P-wavesegment. Therefore, gaining the signal is critical to visually depictingP-wave detail. This technique works most efficaciously with a raw signalwith low noise and high resolution, as generated by the monitor recorder14. Automatic gain control applied to a high noise signal will onlyexacerbate noise content and be self-defeating.

Finally, the download station can include filtering modules specificallyintended to enhance P-wave content. For instance, a P-wave base boostfilter 146, which is a form of pre-emphasis filter, can be applied tothe signal to restore missing frequency content or to correct phasedistortion. Still other filters and types of signal processing arepossible.

Conventional ECG monitors, like Holter monitors, invariably requirespecialized training on proper placement of leads and on the operationof recording apparatuses, plus support equipment purpose-built toretrieve, convert, and store ECG monitoring data. In contrast, theelectrocardiography monitor 12 simplifies monitoring from end to end,starting with placement, then with use, and finally with data retrieval.FIGS. 16A-C are functional block diagrams respectively showing practicaluses 150, 160, 170 of the extended wear electrocardiography monitors 12of FIGS. 1 and 2. The combination of a flexible extended wear electrodepatch and a removable reusable (or single use) monitor recorder empowersphysicians and patients alike with the ability to readily performlong-term ambulatory monitoring of the ECG and physiology.

Especially when compared to existing Holter-type monitors and monitoringpatches placed in the upper pectoral region, the electrocardiographymonitor 12 offers superior patient comfort, convenience anduser-friendliness. To start, the electrode patch 15 is specificallydesigned for ease of use by a patient (or caregiver); assistance byprofessional medical personnel is not required. Moreover, the patient isfree to replace the electrode patch 15 at any time and need not wait fora doctor's appointment to have a new electrode patch 15 placed. Inaddition, the monitor recorder 14 operates automatically and the patientonly need snap the monitor recorder 14 into place on the electrode patch15 to initiate ECG monitoring. Thus, the synergistic combination of theelectrode patch 15 and monitor recorder 14 makes the use of theelectrocardiography monitor 12 a reliable and virtually foolproof way tomonitor a patient's ECG and physiology for an extended, or evenopen-ended, period of time.

In simplest form, extended wear monitoring can be performed by using thesame monitor recorder 14 inserted into a succession of fresh newelectrode patches 15. As needed, the electrode patch 15 can be replacedby the patient (or caregiver) with a fresh new electrode patch 15throughout the overall monitoring period. Referring first to FIG. 16A,at the outset of monitoring, a patient adheres a new electrode patch 15in a location at the sternal midline 16 (or immediately to either sideof the sternum 13) oriented top-to-bottom (step 151). The placement ofthe wearable monitor in a location at the sternal midline (orimmediately to either side of the sternum), with its unique narrow“hourglass”-like shape, significantly improves the ability of thewearable monitor to cutaneously sense cardiac electrical potentialsignals, particularly the P-wave (or atrial activity) and, to a lesserextent, the QRS interval signals indicating ventricular activity in theECG waveforms.

Placement involves simply adhering the electrode patch 15 on the skinalong the sternal midline 16 (or immediately to either side of thesternum 13). Patients can easily be taught to find the physicallandmarks on the body necessary for proper placement of the electrodepatch 15. The physical landmarks are locations on the surface of thebody that are already familiar to patients, including the inter-mammarycleft between the breasts above the manubrium (particularly easilylocatable by women and gynecomastic men), the sternal notch immediatelyabove the manubrium, and the Xiphoid process located at the bottom ofthe sternum. Empowering patients with the knowledge to place theelectrode patch 15 in the right place ensures that the ECG electrodeswill be correctly positioned on the skin, no matter the number of timesthat the electrode patch 15 is replaced.

A monitor recorder 14 is snapped into the non-conductive receptacle 25on the outward-facing surface of the electrode patch 15 (step 152). Themonitor recorder 14 draws power externally from a battery provided inthe non-conductive receptacle 25. In addition, the battery is replacedeach time that a fresh new electrode patch 15 is placed on the skin,which ensures that the monitor recorder 14 is always operating with afresh power supply and minimizing the chances of a loss of monitoringcontinuity due to a depleted battery source.

By default, the monitor recorder 14 automatically initiates monitoringupon sensing body surface potentials through the pair of ECG electrodes(step 153). In a further embodiment, the monitor recorder 14 can beconfigured for manual operation, such as by using the tactile feedbackbutton 66 on the outside of the sealed housing 50, or otheruser-operable control. In an even further embodiment, the monitorrecorder 14 can be configured for remotely-controlled operation byequipping the monitor recorder 14 with a wireless transceiver, such asdescribed in commonly-assigned U.S. Patent application, entitled “RemoteInterfacing of an Extended Wear Electrocardiography and PhysiologicalSensor Monitor,” Ser. No. 14/082,071, filed Nov. 15, 2013, pending, thedisclosure of which is incorporated by reference. The wirelesstransceiver allows wearable or mobile communications devices towirelessly interface with the monitor recorder 14.

A key feature of the extended wear electrocardiography monitor 12 is theability to monitor ECG and physiological data for an extended period oftime, which can be well in excess of the 14 days currently pitched asbeing achievable by conventional ECG monitoring approaches. In a furtherembodiment, ECG monitoring can even be performed over an open-ended timeperiod, as further explained infra. The monitor recorder 14 is reusableand, if so desired, can be transferred to successive electrode patches15 to ensure continuity of monitoring. At any point during ECGmonitoring, a patient (or caregiver) can remove the monitor recorder 14(step 154) and replace the electrode patch 15 currently being worn witha fresh new electrode patch 15 (step 151). The electrode patch 15 mayneed to be replaced for any number of reasons. For instance, theelectrode patch 15 may be starting to come off after a period of wear orthe patient may have skin that is susceptible to itching or irritation.The wearing of ECG electrodes can aggravate such skin conditions. Thus,a patient may want or need to periodically remove or replace ECGelectrodes during a long-term ECG monitoring period, whether to replacea dislodged electrode, reestablish better adhesion, alleviate itching orirritation, allow for cleansing of the skin, allow for showering andexercise, or for other purpose.

Following replacement, the monitor recorder 14 is again snapped into theelectrode patch 15 (step 152) and monitoring resumes (step 153). Theability to transfer the same monitor recorder 14 to successive electrodepatches 15 during a period of extended wear monitoring is advantageousnot to just diagnose cardiac rhythm disorders and other physiologicalevents of potential concern, but to do extremely long term monitoring,such as following up on cardiac surgery, ablation procedures, or medicaldevice implantation. In these cases, several weeks of monitoring or moremay be needed. In addition, some IMDs, such as pacemakers or implantablecardioverter defibrillators, incorporate a loop recorder that willcapture cardiac events over a fixed time window. If the telemetryrecorded by the IMD is not downloaded in time, cardiac events thatoccurred at a time preceding the fixed time window will be overwrittenby the IMD and therefore lost. The monitor recorder 14 providescontinuity of monitoring that acts to prevent loss of cardiac eventdata. In a further embodiment, the firmware executed by themicrocontroller 61 of the monitor recorder 14 can be optimized forminimal power consumption and additional flash memory for storingmonitoring data can be added to achieve a multi-week monitor recorder 14that can be snapped into a fresh new electrode patch 15 every sevendays, or other interval, for weeks or even months on end.

Upon the conclusion of monitoring, the monitor recorder 14 is removed(step 154) and recorded ECG and physiological telemetry are downloaded(step 155). For instance, a download station can be physicallyinterfaced to the external connector 65 of the monitor recorder 14 toinitiate and conduct downloading, as described supra with reference toFIG. 15.

In a further embodiment, the monitoring period can be of indeterminateduration. Referring next to FIG. 16B, a similar series of operations arefollowed with respect to replacement of electrode patches 15,reinsertion of the same monitor recorder 14, and eventual download ofECG and physiological telemetry (steps 161-165), as described supra withreference to FIG. 16A. However, the flash memory 62 (shown in FIG. 9) inthe circuitry 60 of the monitor recorder 14 has a finite capacity.Following successful downloading of stored data, the flash memory 62 canbe cleared to restore storage capacity and monitoring can resume oncemore, either by first adhering a new electrode patch 15 (step 161) or bysnapping the monitor recorder 14 into an already-adhered electrode patch15 (step 162). The foregoing expanded series of operations, to includereuse of the same monitor recorder 14 following data download, allowsmonitoring to continue indefinitely and without the kinds ofinterruptions that often affect conventional approaches, including theretrieval of monitoring data only by first making an appointment with amedical professional.

In a still further embodiment, when the monitor recorder 14 is equippedwith a wireless transceiver, the use of a download station can beskipped. Referring last to FIG. 16C, a similar series of operations arefollowed with respect to replacement of electrode patches 15 andreinsertion of the same monitor recorder 14 (steps 171-174), asdescribed supra with reference to FIG. 16A. However, recorded ECG andphysiological telemetry are downloaded wirelessly (step 175), such asdescribed in commonly-assigned U.S. patent application Ser. No.14/082,071, cited supra. The recorded ECG and physiological telemetrycan even be downloaded wirelessly directly from a monitor recorder 14during monitoring while still snapped into the non-conductive receptacle25 on the electrode patch 15. The wireless interfacing enablesmonitoring to continue for an open-ended period of time, as thedownloading of the recorded ECG and physiological telemetry willcontinually free up onboard storage space. Further, wireless interfacingsimplifies patient use, as the patient (or caregiver) only need worryabout placing (and replacing) electrode patches 15 and inserting themonitor recorder 14. Still other forms of practical use of the extendedwear electrocardiography monitors 12 are possible.

The circuit trace and ECG electrodes components of the electrode patch15 can be structurally simplified. In a still further embodiment, theflexible circuit 32 (shown in FIG. 5) and distal ECG electrode 38 andproximal ECG electrode 39 (shown in FIG. 6) are replaced with a pair ofinterlaced flexile wires. The interlacing of flexile wires through theflexible backing 20 reduces both manufacturing costs and environmentalimpact, as further described infra. The flexible circuit and ECGelectrodes are replaced with a pair of flexile wires that serve as bothelectrode circuit traces and electrode signal pickups. FIG. 17 is aperspective view 180 of an extended wear electrode patch 15 with aflexile wire electrode assembly in accordance with a still furtherembodiment. The flexible backing 20 maintains the unique narrow“hourglass”-like shape that aids long term extended wear, particularlyin women, as described supra with reference to FIG. 4. For clarity, thenon-conductive receptacle 25 is omitted to show the exposed batteryprinted circuit board 182 that is adhered underneath the non-conductivereceptacle 25 to the proximal end 31 of the flexible backing 20. Insteadof employing flexible circuits, a pair of flexile wires are separatelyinterlaced or sewn into the flexible backing 20 to serve as circuitconnections for an anode electrode lead and for a cathode electrodelead.

To form a distal electrode assembly, a distal wire 181 is interlacedinto the distal end 30 of the flexible backing 20, continues along anaxial path through the narrow longitudinal midsection of the elongatedstrip, and electrically connects to the battery printed circuit board182 on the proximal end 31 of the flexible backing 20. The distal wire181 is connected to the battery printed circuit board 182 by strippingthe distal wire 181 of insulation, if applicable, and interlacing orsewing the uninsulated end of the distal wire 181 directly into anexposed circuit trace 183. The distal wire-to-battery printed circuitboard connection can be made, for instance, by back stitching the distalwire 181 back and forth across the edge of the battery printed circuitboard 182. Similarly, to form a proximal electrode assembly, a proximalwire (not shown) is interlaced into the proximal end 31 of the flexiblebacking 20. The proximal wire is connected to the battery printedcircuit board 182 by stripping the proximal wire of insulation, ifapplicable, and interlacing or sewing the uninsulated end of theproximal wire directly into an exposed circuit trace 184. The resultingflexile wire connections both establish electrical connections and helpto affix the battery printed circuit board 182 to the flexible backing20.

The battery printed circuit board 182 is provided with a batterycompartment 36. A set of electrical pads 34 are formed on the batteryprinted circuit board 182. The electrical pads 34 electrically interfacethe battery printed circuit board 182 with a monitor recorder 14 whenfitted into the non-conductive receptacle 25. The battery compartment 36contains a spring 185 and a clasp 186, or similar assembly, to hold abattery (not shown) in place and electrically interfaces the battery tothe electrical pads 34 through a pair battery leads 187 for powering theelectrocardiography monitor 14. Other types of battery compartment arepossible. The battery contained within the battery compartment 36 can bereplaceable, rechargeable, or disposable.

In a yet further embodiment, the circuit board and non-conductivereceptacle 25 are replaced by a combined housing that includes a batterycompartment and a plurality of electrical pads. The housing can beaffixed to the proximal end of the elongated strip through theinterlacing or sewing of the flexile wires or other wires or threads.

The core of the flexile wires may be made from a solid, stranded, orbraided conductive metal or metal compounds. In general, a solid wirewill be less flexible than a stranded wire with the same totalcross-sectional area, but will provide more mechanical rigidity than thestranded wire. The conductive core may be copper, aluminum, silver, orother material. The pair of the flexile wires may be provided asinsulated wire. In one embodiment, the flexile wires are made from amagnet wire from Belden Cable, catalogue number 8051, with a solid coreof AWG 22 with bare copper as conductor material and insulated bypolyurethane or nylon. Still other types of flexile wires are possible.In a further embodiment, conductive ink or graphene can be used to printelectrical connections, either in combination with or in place of theflexile wires.

In a still further embodiment, the flexile wires are uninsulated. FIG.18 is perspective view of the flexile wire electrode assembly from FIG.17, with a layer of insulating material 189 shielding a bare uninsulateddistal wire 181 around the midsection on the contact side of theflexible backing. On the contact side of the proximal and distal ends ofthe flexible backing, only the portions of the flexile wires serving aselectrode signal pickups are electrically exposed and the rest of theflexile wire on the contact side outside of the proximal and distal endsare shielded from electrical contact. The bare uninsulated distal wire181 may be insulated using a layer of plastic, rubber-like polymers, orvarnish, or by an additional layer of gauze or adhesive (ornon-adhesive) gel. The bare uninsulated wire 181 on the non-contact sideof the flexible backing may be insulated or can simply be leftuninsulated.

Both end portions of the pair of flexile wires are typically placeduninsulated on the contact surface of the flexible backing 20 to form apair of electrode signal pickups. FIG. 19 is a bottom view 190 of theflexile wire electrode assembly as shown in FIG. 17. When adhered to theskin during use, the uninsulated end portions of the distal wire 181 andthe proximal wire 191 enable the monitor recorder 14 to measure dermalelectrical potential differentials. At the proximal and distal ends ofthe flexible backing 20, the uninsulated end portions of the flexilewires may be configured into an appropriate pattern to provide anelectrode signal pickup, which would typically be a spiral shape formedby guiding the flexile wire along an inwardly spiraling pattern. Thesurface area of the electrode pickups can also be variable, such as byselectively removing some or all of the insulation on the contactsurface. For example, an electrode signal pickup arranged by sewinginsulated flexile wire in a spiral pattern could have a crescent-shapedcutout of uninsulated flexile wire facing towards the signal source.

In a still yet further embodiment, the flexile wires are left freelyriding on the contact surfaces on the distal and proximal ends of theflexible backing, rather than being interlaced into the ends of theflexible backing 20. FIG. 20 is a bottom view 200 of a flexile wireelectrode assembly in accordance with a still yet further embodiment.The distal wire 181 is interlaced onto the midsection and extends anexposed end portion 192 onto the distal end 30. The proximal wire 191extends an exposed end portion 193 onto the proximal end 31. The exposedend portions 192 and 193, not shielded with insulation, are furtherembedded within an electrically conductive adhesive 201. The adhesive201 makes contact to skin during use and conducts skin electricalpotentials to the monitor recorder 14 (not shown) via the flexile wires.The adhesive 201 can be formed from electrically conductive,non-irritating adhesive, such as hydrocolloid.

The distal wire 181 is interlaced or sewn through the longitudinalmidsection of the flexible backing 20 and takes the place of theflexible circuit 32. FIG. 21 is a perspective view showing thelongitudinal midsection of the flexible backing of the electrodeassembly from FIG. 17. Various stitching patterns may be adopted toprovide a proper combination of rigidity and flexibility. In simplestform, the distal wire 181 can be manually threaded through a pluralityof holes provided at regularly-spaced intervals along an axial pathdefined between the battery printed circuit board 182 (not shown) andthe distal end 30 of the flexible backing 20. The distal wire 181 can bethreaded through the plurality of holes by stitching the flexile wire asa single “thread.” Other types of stitching patterns or stitching ofmultiple “threads” could also be used, as well as using a sewing machineor similar device to machine-stitch the distal wire 181 into place, asfurther described infra. Further, the path of the distal wire 181 neednot be limited to a straight line from the distal to the proximal end ofthe flexible backing 20.

An effective ECG compression solution can reduce battery powerconsumption, ameliorate storage restriction, and extend monitoring time.The effectiveness of an ECG compression technique is evaluated mainlythrough compression ratio, degree of error loss, and execution time. Thecompression ratio is the ratio between the bit rate of the originalsignal and the bit rate of the compressed one. The error loss is theerror and loss in the reconstructed data compared to non-compresseddata. The execution time is the computer processing time required forcompression and decompression. A lossless compressions may provide exactreconstruction of ECG data, but usually cannot provide a significantcompression ratio, thus may not be a good choice when high compressionratio is required. In addition, analysis of ECG data does not requireexact reconstruction; only certain feature of the ECG signal areactually important. Therefore, lossy compression, or techniques thatintroduce some error in the reconstructed data, is useful because lossycompression may achieve high compression ratios.

The ECG signal captured by the monitor recorder 14 is compressed by thecompression module 134 as part a firmware 132 located on microcontroller61 prior to being outputted for storage, as shown in FIG. 14. FIG. 22 isa flow diagram showing a monitor recorder-implemented method for ECGsignal processing and ECG data compressing for use in the monitorrecorders of FIG. 4. A series of ECG signals are sensed through thefront end circuit 63, which converts analog ECG signals into anuncompressed digital representation. The compressing module 134 firstread the digital presentation of the ECG signals or ECG values (step201). The compressing module 134 subsequently encodes the ECG value(step 202). This encoding step achieves one level of compression and inone embodiment is a lossy compression, as further discussed infra inFIG. 23. The compressing module 134 also perform a second level ofcompression by further encoding and compressing the sequence of codesresulting from the encoding process of step 202 (step 203). Thecompressed data is stored into a non-volatile memory, such as the flashmemory 62.

Monitoring ECG (step 201) is described in FIG. 12. Encoding ECG values(step 202) is performed by translating each sample data into one ofcodes, or encodings, further described with reference to in FIG. 24. Byencoding ECG data in the form of a series of codes, a level ofcompression is achieved. FIG. 23 is a flow diagram showing a monitorrecorder-implemented method for encoding ECG values. FIG. 24 is anexample of a panel of codes or encodings with each code covering a rangedefined by a lower threshold ECG value and an upper threshold ECG value,to be referenced to during the encoding process described in FIG. 23. Inone embodiment, a series of ECG values are obtained, which constitute adatastream (step 211). The series of ECG value can be one of rawelectrocardiography value, processed electrocardiography value, filteredelectrocardiography value, averaged electrocardiography value, orsampled electrocardiography value. The compression module 134 defines aplurality of bins, each bin comprising a lower threshold ECG value, anupper threshold ECG value, and an encoding or code (step 212). Oneexample of such a panel of the bins is shown in the Table in FIG. 24,with the first column denoting the lower threshold ECG value, the secondcolumn denoting the upper threshold ECG value, and the third columndenoting the code of a bin. An ECG data value is assigned to acorresponding bin, based upon the difference between the data value anda serial accumulator. The first serial accumulator is set to apre-determined value such as a center value of an ECG recorder (step213), each succeeding serial accumulator is a function of a previousserial accumulator and the actual ECG reading and will be describedinfra. For the series of the ECG values, the following encoding stepsare performed by the compression module (steps 214 to 220). These stepsincludes: selecting the ECG value next remaining in the series to beprocessed (step 215); taking a difference between the selected ECG valueand the serial accumulator (step 216); identifying the bin in theplurality of the bins corresponding to the difference (step 217), whichwill be further described infra; representing the selected ECG value bythe encoding for the identified bin (218); and adjusting the serialaccumulator by a value derived from the identified bin (step 219).Through this process, each ECG value is represented, or encoded, by oneof the bins. As a result, one level of data compression is achievedsince the limited number of bins requires less storage space compared tothe actual ECG data values.

Several ways of executing the step 217, i.e., identifying the bin in theplurality of the bins corresponding to the difference between theselected ECG value and the serial accumulator, or assigning a differencebetween the selected ECG value and the serial accumulator to a properbin. In one embodiment, a difference is assigned to a bin when thedifference lies between the lower threshold ECG value and the upperthreshold ECG value of the bin. There are two options to assign a binwhen a difference between the selected ECG value and the serialaccumulator falls onto the lower threshold ECG value or the upperthreshold ECG value of a bin. In one option, a bin is identified whenthe difference is equal to or larger than the lower threshold ECG valueand smaller than the upper threshold ECG value of the identified bin. Inthe other option, a bin is identified when the difference is larger thanthe lower threshold ECG value and equal to or smaller than the upperthreshold ECG value of the identified bin.

During the step 219, the value derived from the identified bin can bethe lower threshold ECG value, the higher threshold ECG value, or anumber derived from the lower threshold ECG value, upper threshold ECGvalue, or both. The derivation can be an addition or subtraction of thelower or upper threshold ECG value by a constant number or an offset.The derivation can also be an adaptive process wherein the offset may beadjusted to input ECG data, and varies from one bin to another bin.

Converting ECG values into a limited numbers of codes facilitate afurther compression step which will be described infra. Some data errorloss is introduced by the encoding process; however, proper bin setupminimizes the ECG data error loss and preserves useful data essentialfor accurate diagnosis, including P-wave signal. The number of codes andthe lower and upper threshold ECG value of the codes are determined toachieve both efficient encoding and sufficient data reconstruction,especially for P-wave signals. The number of codes and the lower andupper threshold ECG value of the codes are flexible and can be adjustedto adapt to ECG data input and storage space. In one embodiment, thenumber of the bins are chosen from 2³ to 2¹⁰. A higher number of binsusually results in less ECG data error loss but more storage space andbattery power use.

The proper demarcation of upper and lower thresholds also reduces errorloss and contributes to accurate re-construction of ECG value and graphshape. The number of bins and the thresholds for these bins arecarefully selected to keep essential information of the ECG signals andfilter away non-essential information, with a special emphasis toaccurately representing the P-wave. Normally, each successive bincontinues forward from a previous bin so as to cover a contiguous rangeof electrocardiography values. In one embodiment, the size of the bins,i.e., the interval between the higher threshold ECG value and the lowerthreshold ECG value, are not equal thought the contiguous range;instead, areas of high frequency calls for a smaller size of bins. Thesize of the bins is partly determined by the frequency of the ECG valuesfalling into the bin.

In one embodiment, 2⁴=16 bins are used, as described with reference toin FIG. 24 where the lower threshold ECG value and upper threshold ECGvalue for each bin are also provided. This setup provides minimum errorloss and a significant compression ratio, among other considerations.The first, second, and third columns represent the lower threshold ECGvalue, the upper threshold ECG value, and the coding of the bins. Thebin that an ECG data will fall into depends on the difference betweenthe raw ECG data value and corresponding serial accumulator compared tothe range that the bin covers. If an ECG raw data falls into aparticular bin, the raw ECG data can be represented by the code of thebin. In this example, the codes are encoded with a four-bit storagespace, with one bit to encode sign and three bits to encode magnitude.Similar, up to 32 codes can be encoded with a five-bit storage space,with one bit to encode sign and 4 bits to encode magnitude.

The minimum (Min) and maximum (Max) values in FIG. 24 defines aninclusive range of ECG values for each ECG code. An input ECG value thatfall within the range defined by Min and Max value will be encoded bythe code in the third column in FIG. 24. The Min and Max ranges can bethe same for all of the bins or can be tailored to specific ranges ofECG values, to emphasize higher or lower density. For example, the Minand Max value 5,001-50,000 correspond to code +7 is low density andreflects the expectation that few actual ECG values exceeding 5001 μVwill occur. The density of the Min and Max value can be adjusted toenhance ECG signal detection such as P-wave signal. as a furtherexample, the Min and Max ECG value ranges can be evenly definedthroughout, or be doubled each of the successive bin.

In one embodiment, the number of bins is selected to be a power of two,although a power of two is not strictly required, particularly when asecond stage compression as further described below with reference toFIG. 26.

FIG. 25 is an example illustrating the encoding and compression schemein accordance with method and parameters as described with reference toin FIGS. 23 and 24. The first three ECG values of an ECG datastream,12000, 11904, and 12537, are shown in column Ito show a recursiveprocess. Remaining values are omitted since they are processed throughthe same recursive process. The initial ECG value, 12000, is equivalentto the center value of the ECG recorder. The initial serial accumulatoris assigned to the center value of the ECG recorder, 12000. Thedifference between the initial ECG value to the initial serialaccumulator is 0, which falls within the lower and upper threshold ofbin 0. Thus the initial ECG value is encoded with the code 0. 12000 istransferred to next row as the serial accumulator for next ECG value.The next ECG value is 11904. The difference between the next ECG valueand the serial accumulator for the second value is 11904−12000=−96. Thedifference of −96 falls into the bin with the code of −3, where thelower threshold of the bin is −41 and the upper threshold of the bin is−150. Thus, the second ECG value is encoded with the code of −3, whichis the bin identification. For the purpose of decoding the second value,an encoder first refers to the assigned bin, which is bin −3; theencoder then reads the lower threshold ECG value of the assigned bin −3,which is −41; and the encoder finally add the lower threshold ECG valueof the assigned bin to the decoded value of the first ECG value, whichis 12000, to arrive at a decoded value of 11959. The decoded value 11959in turn serves as the serial accumulator for the next ECG value, in thiscase the next ECG value is the third one of 12537. The differencebetween the third value and its corresponding serial accumulator is12537−11959=578. This difference, 578, falls into the bin with a code of+5, which has a lower threshold ECG value of 301 and upper threshold ECGvalue of 1500. Thus the third ECG value is encoded with the code of +5.The third ECG value is decoded by adding the lower threshed ECG value ofthe assigned bin +5, which is 301, to the decoded value of second ECGvalue, which is 11959, to arrive at the decoded value of 12260. Thedecoded value of 12260 in turn will serve as the serial accumulator forthe next ECG value. The encoding process continue until the last readingis taken. The encoder keeps track of the accumulated encoded value asthe encoding process progresses along.

The encoding process described above is also a lossy compression processthat encodes raw ECG signals with a finite number of codes. This processcaptures essential information while achieving significant datacompression. In one embodiment, an other compressing step is performed.The other compression step may be performed independently. The othercompression step may also be performed on top of the encoding processdescribed above to achieve a higher level compression than one stepalone. The second compression step can be a lossless compressionperformed on the codes from the first step. In one embodiment, thecompression ratio of the second compression is in the range of 1.4 to1.6, increasing the data storage capacity of a non-volatile memory bymore than 41-66%. In another embodiment, the compression ratio of thesecond compression is in excess of 1.6, increasing the data storagecapacity of a non-volatile memory by more than 66%. Thus, thecombination of the lossy compression and the lossless compression servesto achieve both high fidelity of the ECG signal preservation and highcompression ratio, which translate into increased data storage capacityand reduced power consumption for the ambulatory electrocardiographymonitor, resulting in extended wear time of the monitor.

In one embodiment, the second compression is effected by encoding asequence of codes obtained from the first compression into a singlenumber between 0 and 1, with frequently used codes using fewer bits andnot-so-frequently occurring codes using more bits, resulting in reducedstorage space use in total. FIG. 26 is a flow diagram showing a monitorrecorder-implemented method for further compressing the codes. Asequence of the codes corresponding to the series of the ECG values isprovided to the compressing module 134. The compressing module 134 set arange of 0 to 1 to an initial sequence of the codes (step 231). Thecompressing module 134 further performs recursive steps of assigningeach successive codes into a sub-range within a previous range accordingto the probabilities of the codes appearing after a code (steps232-239). In order to do so, the compressing module 134 obtains anestimation of probabilities of next codes, given a current code (step233). Several variations of calculating and adjusting the probabilitiesof the next codes will be described infra. The compressing module 134divides the range of the current code into sub-ranges, each sub-rangerepresenting a fraction of the range proportional to the probabilitiesof the next codes (step 234). These sub-ranges are contiguous andsequential. The compressing module 134 reads the next code (step 235)and selects the sub-range corresponding to the read next code (step236). The read next code is represented, or encoded, by thecorresponding sub-range (step 237). The sub-range corresponding to theread next code is assigned to be the range for the code next to the readnext code (step 238), and the range is further divided into sub-rangeswith each sub-range representing a fraction of the range proportional tothe probabilities of codes next to the read next code (step 39). In thisway, each code in the sequence of the codes is represented by, orencoded through, its location within a sub-range through a recursiveprocess. During the recursive process, strings of codes represented bythe selected sub-ranges are encoded into part of the single numberbetween 0 and 1 and can be periodically or continually stored into thenon-volatile memory, can be stored on-demand or as-needed, or can bequeued up and stored en masse upon completion of the process. Oneexample of the non-volatile memory is the flash memory 62.

The compressing module 134 uses a statistical model to predict what thenext code is, given a current encoding (Step 233). In one embodiment, atotal of 16 codes or bin numbers are used, thus the statistical modeluses 16 tables, one for each current code. Within each table, numericpossibilities for 16 possible next codes given the particular currentcode are generated. In one embodiment, the probabilities of the nextcodes can be calculated from sample ECG values. In another embodiment,the probabilities of the next codes can be modified by ECG dataincluding recorded ECG data and data presently recorded. In stillanother embodiment, the probabilities of next codes can be adaptive,i.e, adjusted or varied along the recursive compression steps. Finally,in yet another embodiment, the compressing module 134 may use astatistical model to arrive at the estimation of probabilities of nextcodes, given two or more consecutive preceding codes. When twoconsecutive preceding codes are used, 16×16=256 different pairs ofconsecutive codes are possible. The compressing module 134 generates 256tables, each tables containing numeric possibilities for 16 possiblenext codes given a particular pair of previous codes. When threeconsecutive preceding codes are used, 16×16×16=4096 different trios ofconsecutive codes are possible. The compressing module 134 generates4096 tables, each tables containing numeric possibilities for 16possible next codes given a particular trio of previous codes. Using twoor more consecutive preceding codes further enhances compression ratiocompared to using one preceding code, but also demands more processingpower from the microcontroller.

While the invention has been particularly shown and described asreferenced to the embodiments thereof, those skilled in the art willunderstand that the foregoing and other changes in form and detail maybe made therein without departing from the spirit and scope.

What is claimed is:
 1. A computer-implemented method for encoding andcompressing electrocardiography values, comprising the steps of:obtaining a series of electrocardiography values, wherein each of theelectrocardiography values can be one of a raw electrocardiographyvalue, processed electrocardiography value, filtered electrocardiographyvalue, averaged electrocardiography value, and a sampledelectrocardiography value; defining a plurality of bins, each bincomprising a lower threshold electrocardiography value, an upperthreshold electrocardiography value, and a code for the bin; setting aserial accumulator to a pre-determined value; processing each of theelectrocardiography values from the series of the electrocardiographyvalues, comprising the steps of: selecting the electrocardiography valuenext remaining to be processed; taking a difference of the selectedelectrocardiography value and the serial accumulator; identifying thebin in the plurality of the bins corresponding to the difference;representing the selected electrocardiography value by the code for theidentified bin; adjusting the serial accumulator by a value derived fromthe identified bin; and writing each of the represented codes into asequence in a non-volatile memory.
 2. A method in accordance with claim1, further comprising the step of encoding the sequence into a singlenumber between 0 and
 1. 3. A method in accordance with claim 2, furthercomprising encoding the sequence by the steps of: setting a range of 0to 1 for an initial code from the sequence of the represented codes; andprocessing each of the codes remaining in the sequence, comprising thesteps of: obtaining an estimation of probabilities of next codes;dividing the range into sub-ranges, each sub-range representing afraction of the range proportional to the probabilities of the nextcodes; obtaining a next code; selecting the sub-range corresponding tothe next code; representing the next code by the selected sub-range; andcontinuing the steps of the process using the selected sub-range inplace of the range.
 4. A method in accordance with claim 3, wherein theprobabilities of the next codes are obtained through a sampleelectrocardiography data or the sequence of the represented codes.
 5. Amethod in accordance with claim 3, further comprising the step ofadjusting the probabilities during at least one of recursive rangedivisions.
 6. A method in accordance with claim 1, wherein the valuederived from the identified bin may be based on one of the lowerthreshold electrocardiography value, the upper thresholdelectrocardiography value, a modification of the lowerelectrocardiography value with a constant number or a variable number,and a modification of the upper threshold electrocardiography value witha constant number or variable number.
 7. A method in accordance withclaim 1, wherein the predetermined value is a center value of anelectrocardiography recorder.
 8. A method in accordance with claim 1,wherein each successive bin is continuing forward from a previous bin soas to cover a contiguous range of the electrocardiography values.
 9. Amethod in accordance with claim 1, wherein the bin is identified whenthe difference is equal to or larger than the lower thresholdelectrocardiography value and smaller than the upper thresholdelectrocardiography value of the identified bin.
 10. A method inaccordance with claim 1, wherein the bin is identified when thedifference is larger than the lower threshold electrocardiography valueand equal to or smaller than the upper threshold electrocardiographyvalue of the identified bin.
 11. A computer-implemented method forencoding and compressing electrocardiography values, comprising thesteps of: obtaining a series of electrocardiography values; defining aplurality of bins, each bin comprising a lower thresholdelectrocardiography value, an upper threshold electrocardiography value,and a code for the bin; setting a serial accumulator to predeterminedvalue; processing each of the electrocardiography values from the seriesof the electrocardiography values, comprising the steps of: selectingthe electrocardiography value next remaining to be processed; taking adifference of the selected electrocardiography value and the serialaccumulator; identifying the bin in the plurality of the binscorresponding to the difference; representing the selectedelectrocardiography value by the code for the identified bin; adjustingthe serial accumulator by a value derived from the identified bin;concatenating each of the represented codes into a sequence; encodingthe sequence of the represented codes into a single number between 0 and1, further comprising the step of: setting a range for an initial codefrom the sequence of the represented codes; and processing each of thecodes remaining in the sequence, comprising the steps of: obtaining anestimation of probabilities of next codes; dividing the range intosub-ranges, each sub-range representing a fraction of the rangeproportional to the probabilities of the next codes; obtaining a nextcode; selecting the sub-range corresponding to the next code;representing the next code by the selected sub-range; and continuing thesteps of the process using the selected sub-range in place of the range;and storing the encoded single number into the non-volatile memory. 12.A method in accordance with claim 11, wherein the probabilities of thenext codes are obtained through a sample electrocardiography data or thesequence of the represented codes.
 13. A method in accordance with claim11, further comprising the step of adjusting the probabilities during atleast one of recursive range divisions.
 14. A method in accordance withclaim 11, wherein the value derived from the identified bin may be basedon one of the lower threshold electrocardiography value, the upperthreshold electrocardiography value, a modification of the lowerelectrocardiography value with a constant number or a variable number,and a modification of the upper threshold electrocardiography value witha constant number or variable number.
 15. A method in accordance withclaim 11, wherein the predetermined value is a center value of anelectrocardiography recorder.
 16. A method in accordance with claim 11,wherein each successive bin is continuing forward from a previous bin soas to cover a contiguous range of the electrocardiography values.
 17. Amethod in accordance with claim 11, wherein the bin is identified whenthe difference is equal to or larger than the lower thresholdelectrocardiography value and smaller than the upper thresholdelectrocardiography value of the identified bin.
 18. A method inaccordance with claim 11, wherein the bin is identified when thedifference is larger than the lower threshold electrocardiography valueand equal to or smaller than the upper threshold electrocardiographyvalue of the identified bin.
 19. A method in accordance with claim 11,wherein the electrocardiography value can be one of rawelectrocardiography value, processed electrocardiography value, filteredelectrocardiography value, averaged value, and sampled value.